1. Field of the Invention
The present invention relates to a method for recovering 2-chloropropionic acid (hereinafter called ".alpha.-CPA") with high purity from crude .alpha.-CPA or a crude .alpha.-CPA-containing solution.
2. Description of the Related Art
.alpha.-CPA is used as an intermediate for various chemical products such as lactic acid and alanine, agricultural chemicals, pharmaceuticals, etc. In recent years, there is a growing demand for its high purity product in the field of a racemic resolution.
.alpha.-CPA is produced by chlorination of propionic acid industrially. Its production is conducted in accordance with the following reaction formula: EQU CH.sub.3 CH.sub.2 COOH+Cl.sub.2 .fwdarw.CH.sub.3 CHClCOOH+HCl(1)
As impurities, more chlorinated dichloro derivatives such as 2,2-dichloropropionic acid are generally contained on the order of several percents in addition to unreacted propionic acid. Moreover, it has heretofore been considered difficult to produce a high-purity product of 99 wt. % or higher by conventional methods. The term "dichloro derivatives" as used herein means chlorinated compounds of propionic acid, in which two hydrogen atoms have been substituted by the like number of chlorine atoms. In general, 2, 2-dichloropropionic acid is a predominant component of the dichloro derivatives. The boiling points of these dichloro derivatives are extremely close to that of .alpha.-CPA so that they can hardly be separated out by conventional distillation. As a matter of fact, none of the other separation methods have been found acceptable from the industrial standpoint.
For these reasons, the purity is around 98-99 wt. % even in the case of .alpha.-CPA currently produced as a high-purity product in the industry. .alpha.-CPA whose purity is 99 wt. % or higher is not produced by the chlorination of propionic acid.
Recently, a process has been proposed for the production of .alpha.-CPA whose purity is as high as 99 wt. % or more (Japanese Patent Laid-Open No. 96446/1987). This process features oxidation of 2-chloropropionaldehyde (hereinafter called ".alpha.-CPL") with oxygen or an oxygen-containing gas in a liquid phase in the presence of a metal compound catalyst.
The production process of .alpha.-CPA by liquid-phase oxidation of .alpha.-CPL proceeds in accordance with the following reaction formula: EQU CH.sub.3 CHClCHO+1/2O.sub.2 .fwdarw.CH.sub.3 CHClCOOH (2)
The starting material, .alpha.-CPL, can be produced by the reaction of vinyl chloride and synthesis gas in the presence of rhodium and a base, for example, as disclosed in Japanese Patent Laid-Open No. 126046/1986. Accordingly, chlorine is not used and almost no byproduction of hardly-separable dichloro derivatives takes place, thereby making it possible to produce high-purity .alpha.-CPA of 99 wt. % or more.
For the removal of dichloro derivatives, Japanese Patent Laid-Open No. 58014/1975 discloses an azeotropic distillation method which makes use of an aliphatic hydrocarbon as an azeotrope former. Even in this process, the difference in boiling point between the azeotropic mixture of the azeotrope former so added and .alpha.-CPA and the azeotropic mixture of the thus-added azeotrope former and each dichloro derivative is however not substantially large. Production of high-purity .alpha.-CPA therefore requires not only a distillation column having an extremely large number of plates or trays and requiring huge energy but also an additional step for the separation and recovery of the azeotrope former.
French Patent 2582645 discloses a method in which hydrogenation is conducted in the presence of a metal catalyst such as palladium, rhodium or ruthenium to convert hardly-separable dichloro derivatives to propionic acid having the lower boiling point, followed by distillation for purification. This method however requires hydrogen. Moreover, hydrogen chloride is byproduced together with propionic acid, thereby making the method unfavorable from the viewpoint of protection of the production facilities from corrosion.
On the other hand, in the production process of high-purity .alpha.-CPA by liquid-phase oxidation of .alpha.-CPL, the reaction mixture contains unreacted .alpha.-CPL, reaction byproducts, a catalyst, a solvent, and so on. Purification by conventional distillation however leads to problems such that .alpha.-CPA cannot be obtained in a colorless, clear form probably due to its poor thermal stability and .alpha.-CPA is lost substantially by modification during the purification.